In nuclear reactors, particularly in pressurized water nuclear reactors, the storage of the irradiated fuel assemblies withdrawn from the reactor core takes place in a storage and cooling pond arranged near the reactor building.
The irradiated fuel assemblies are immersed in the pond water and are submerged under a sufficient depth of water to provide biological protection of the regions situated in the vicinity of the storage pond.
In order to facilitate the storage of the fuel assemblies and to increase the capacity of the storage ponds, the fuel assemblies taken out of the reactor core are arranged in casings or cells defining a housing whose shape corresponds to the shape of the fuel assembly and which are arranged according to a uniform pattern to form a storage rack for fuel assemblies. The walls of the various casings or cells forming a storage rack are placed parallel to one another and slightly apart, so as to decrease the volume of the storage rack and hence to increase the storage capacity of the pond, while retaining the possibility of cooling the storage assemblies by means of sheets of water in contact with the cell walls.
To limit the activity and the heating of the stored fuel assemblies while retaining a small separation between the cells, the walls of the cells are partially made of a material having a high neutron-absorption capacity. The neutron-absorbing products or neutron absorbers most frequently used are boron carbide and cadmium. These neutron absorbers are generallly in the form of a layer arranged between two stainless steel walls forming the inner and outer faces of the cell wall.
In this way, the neutron absorber does not come into contact with the pond water inasmuch as the stainless steel walls surrounding this material are fastened to one another at their ends, so as to form a leakproof enclosure. These walls, which protect the neutron absorber against mechanical impacts which can occur in the course of handling operations are therefore also responsible for protecting this material against corrosion.
The cells are made and constructed with great care, so as to endow the jacket enclosing the neutron absorber and forming the wall of these cells with the best possible leakproofing. However, this leakproofing cannot be guaranteed during extended periods of use when the cells are continuously immersed in the water of the cooling pond. Furthermore, it is possible for certain parts of the walls of the cells to become damaged and to lose their leakproofing in the course of the handling operations on the cells themselves or on the nuclear fuel.
In this case, the pond water comes into contact with the neutron absorber and may cause more or less extensive damage to the neutron absorption layer in the cell wall. This may result locally in an activation and abnormal heating of the fuel assemblies. The fuel assemblies must remain in the storage pond for a time sufficient to reduce the residual activity of the assemblies sufficiently to allow these assemblies to be transported to the reprocessing or definitive storage sites. After a residence time in the pond water which may be of long duration, there is a risk that the storage cell no longer provides the safety guarantees resulting from the presence of continuous neutron-absorbent walls surrounding the fuel cells.
In the case where abnormal behavior of a storage rack is detected, it may be necessary to perform a complete change of such storage rack after unloading the fuel assemblies. To avoid such a complex operation, provision is generally made for racks comprising individually removable cells which can be replaced in the storage rack which remains in place in the cooling pond.
However, there has so far been no known process or device making it possible to monitor or supervise the state of the absorbent walls of the fuel assembly storage cells in a cooling pond.